Doping of semiconductor fin devices

ABSTRACT

A semiconductor structure includes of a plurality of semiconductor fins overlying an insulator layer, a gate dielectric overlying a portion of said semiconductor fin, and a gate electrode overlying the gate dielectric. Each of the semiconductor fins has a top surface, a first sidewall surface, and a second sidewall surface. Dopant ions are implanted at a first angle (e.g., greater than about 7°) with respect to the normal of the top surface of the semiconductor fin to dope the first sidewall surface and the top surface. Further dopant ions are implanted with respect to the normal of the top surface of the semiconductor fin to dope the second sidewall surface and the top surface.

This application is a continuation of patent application Ser. No.10/425,156, entitled “Doping of Semiconductor Fin Devices,” filed onApr. 29, 2003, which application is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to semiconductor devices and moreparticularly to semiconductor devices with fin structures and methodsfor doping semiconductor fin devices.

BACKGROUND

The dominant semiconductor technology used for the manufacture ofultra-large scale integrated (ULSI) circuits is themetal-oxide-semiconductor field effect transistor (MOSFET) technology.Reduction in the size of MOSFETs has provided continued improvement inspeed performance, circuit density, and cost per unit function over thepast few decades. As the gate length of the conventional bulk MOSFET isreduced, the source and drain increasingly interact with the channel andgain influence on the channel potential. Consequently, a transistor witha short gate length suffers from problems related to the inability ofthe gate to substantially control the on and off states of the channel.

Phenomena such as reduced gate control associated with transistors withshort channel lengths are termed short-channel effects. Increased bodydoping concentration, reduced gate oxide thickness, and ultra-shallowsource/drain junctions are ways to suppress short-channel effects.However, for device scaling well into the sub-50 nm regime, therequirements for body-doping concentration, gate oxide thickness, andsource/drain (S/D) doping profiles become increasingly difficult to meetwhen conventional device structures based on bulk silicon (Si)substrates are employed. Innovations in front-end process technologiesor the introduction of alternative device structures may be needed tosustain the historical pace of device scaling.

For device scaling well into the sub-30-nm regime, a promising approachto controlling short-channel effects is to use an alternative transistorstructure with more than one gate, i.e., multiple-gates. An example ofthe alternative transistor structure is the multiple-gate transistor.Examples of the multiple-gate transistor include the double-gatetransistor, triple-gate transistor, omega field-effect transistor (FET),and the surround-gate or wrap-around gate transistor. A multiple-gatetransistor structure is expected to extend the scalability of CMOStechnology beyond the limitations of the conventional bulk MOSFET andrealize the ultimate limit of silicon MOSFETs. The introduction ofadditional gates improves the capacitance coupling between the gates andthe channel, increases the control of the channel potential by the gate,helps suppress short channel effects, and prolongs the scalability ofthe MOS transistor.

The simplest example of a multiple-gate transistor is the double-gatetransistor, as described in U.S. Pat. No. 6,413,802 ('802) issued to Hu,et al. and incorporated herein by reference. In the '802 patent, thetransistor channel comprises a thin silicon fin defined using an etchantmask and formed on an insulator layer, e.g., silicon oxide. Gateoxidation is performed, followed by gate deposition and gate patterningto form a double-gate structure overlying the sides of the fin. Both thesource-to-drain direction and the gate-to-gate direction are in theplane of the substrate surface.

SUMMARY OF THE INVENTION

The preferred embodiment of the present invention provides severalmethods for doping the semiconductor fin in a multiple-gate transistorto provide improved performance. In embodiments of the invention, thechannel length is more uniformly doped than in certain prior artimplementation thereby improving performance.

A first embodiment provides a method of doping semiconductor fins ofmultiple-gate transistors. A semiconductor structure includes of aplurality of semiconductor fins overlying an insulator layer, a gatedielectric overlying a portion of said semiconductor fin, and a gateelectrode overlying the gate dielectric. Each of the semiconductor finshas a top surface, a first sidewall surface, and a second sidewallsurface. Dopant ions are implanted at a first angle (greater than 7″)with respect to the normal of the top surface of the semiconductor finto dope the first sidewall surface and the top surface. Further dopantions are implanted with respect to the normal of the top surface of thesemiconductor fin to dope the second sidewall surface and the topsurface.

A second embodiment provides a method to dope semiconductor fins with adifferent orientation. In this embodiment, a first mask covers thesecond semiconductor fin while the first semiconductor fin is doped byimplanting dopant ions with a large implant angle. Similarly, a secondmask covers the first semiconductor fin while the second semiconductorfin is doped by implanting the dopant ions with a large implant angle.

The present invention also includes structure embodiments. For example,semiconductor-on-insulator chip includes a plurality of multiple-gatetransistors formed on an insulator layer. Each multiple-gate transistorincludes a semiconductor fin having an orientation and a gate electrodehaving a gate length of less than 30 nm. The orientation of eachtransistor of the plurality of multiple-gate transistors is the same.Other methods and structures are also taught.

A semiconductor-on-insulator chip comprising of a plurality ofmultiple-gate transistors formed on an insulator layer, eachmultiple-gate transistor comprising of a semiconductor fin having anorientation and a gate electrode having a gate length equal to theminimum feature size, said orientations of the plurality ofmultiple-gate transistors being the same.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawing, in which:

FIG. 1 a illustrates a double gate device structure;

FIG. 1 b illustrates a triple-gate device structure;

FIG. 1 c illustrates an omega field-effect transistor;

FIG. 2 is a plan view of a multiple gate transistor;

FIG. 3 a illustrates a three-dimensional perspective of a triple-gatetransistor;

FIG. 3 b illustrates a three-dimensional perspective of the omega FET;

FIG. 4 a is a cross-sectional view taken along B-B′ of FIG. 2;

FIG. 4 b is a cross-sectional view taken along C—C′ of FIG. 2;

FIGS. 5 a, 5 b and 5 c are cross-sectional views showing doping of asemiconductor fin using a large angle implant;

FIG. 6 a illustrates the doses received in the top and sidewall surfaceregions of the semiconductor fin as a function of the implant angle;

FIG. 6 b illustrates the ratio of the dose in the top region to the dosein a sidewall surface region as a function of the implant angle;

FIG. 7 a is a plan view of the semiconductor fin device of the presentinvention;

FIG. 7 b is a cross-sectional view of the semiconductor fin device ofthe present invention;

FIG. 8 a illustrates the second transistor masked during the doping ofthe source and drain regions of the first transistor;

FIG. 8 b illustrates the first transistor masked during the doping ofthe source and drain regions of the second transistor; and

FIG. 9 illustrates an n-type transistor of the present invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The preferred embodiment of the present invention relates to the fieldof semiconductor devices and more particularly to semiconductor deviceswith a fin structure. The present invention provides several methods fordoping the semiconductor fin in a multiple-gate transistor.

Another example of the multiple-gate transistor is the triple-gatetransistor 102. The cross-section of the triple-gate transistorstructure 102 is illustrated in FIG. 1 b. The plan view of thetriple-gate structure is the same as the double-gate structure and isshown in FIG. 2. The triple-gate transistor structure has a gateelectrode 110 that forms three gates: one gate on the top surface 124 ofthe silicon body/fin 112, and two gates on the sidewalls 114 of thesilicon body/fin 112. The triple-gate transistor achieves better gatecontrol than the double-gate transistor because it has one more gate onthe top of the silicon fin. A three-dimensional view of the triple-gatetransistor 102 is shown in FIG. 3 a. FIG. 3 a illustrates source region126 and drain region 128, which are formed in silicon body 112 onopposite sides of the channel region. An example of a triple-gatetransistor is provided by R. Chau, et al., “Advanced depleted-substratetransistors: single-gate, double-gate, and tri-gate,” 2002 InternationalConference on Solid State Devices and Materials, Nagoya, Japan, pp.68-69, September 2002, which is incorporated herein by reference.

The triple-gate transistor structure may be modified for improved gatecontrol, as illustrated in FIG. 1 c. Such a structure 104 is also knownas the Omega (W) field-effect transistor (FET), or simply omega-FET,since the gate electrode 110 has an omega-shape in its cross-sectionalview. The encroachment of the gate electrode 110 under the semiconductorfin or body 112 forms an omega-shaped gate structure. It closelyresembles the Gate-All-Around (GAA) transistor for excellentscalability, and uses a very manufacturable process similar to that ofthe double-gate or triple-gate transistor. The omega-FET has a top gateadjacent surface 124, two sidewall gates adjacent surfaces 114, andspecial gate extensions or encroachments 130 under the fin-likesemiconductor body.

The omega-FET is therefore a field effect transistor with a gate 110that almost wraps around the body 112. In fact, the longer the gateextension 130, i.e., the greater the extent of the encroachment E, themore the structure approaches or resembles the gate-all-aroundstructure. A three-dimensional perspective of the triple-gate transistorwith recessed insulator, or omega-FET, is schematically illustrated inFIG. 3 b. The encroachment of the gate electrode 110 under the siliconbody 112 helps to shield the channel from electric field lines from thedrain 128 and improves gate-to-channel controllability, thus alleviatingthe drain-induced barrier lowering effect and improving short-channelperformance. An example of an omega field-effect transistor is providedby F. L. Yang, et al., “25 nm CMOS Omega-FET's,” International ElectronDevice Meeting, Dig., Technical Papers, December 2002, which isincorporated herein by reference.

Another multiple-gate device that can utilize aspects of the presentinvention is the surround-gate of wrap-around gate transistor mentionedabove. Examples of these deices are taught in the following references,each of which is incorporated herein by reference: J. P. Colinge, etal., silicon-on-insulator gate-all-around device,” InternationalElectron Device Meeting, Dig. Technical Papers, pp. 595-598, December1990; U.S. Pat. No. 6,391,782, B. Yu, Advanced Micro Devices, Inc., May21, 2002, “Process for forming multiple active lines and gate-all-aroundMOSFET; E. Leobandung, et al., “Wire-Channel and wrap-around-gatemetal-oxide-semiconductor field-effect transistors with a significantreduction of short channel effects,” J. Vacuum Science and Technology B,vol. 15, no. 6, pp. 2791-2794, 1997; and U.S. Pat. No. 6,451,656, B. Yu,et al., Advanced Micro Devices, Inc., Sep. 17, 2002, “CMOS inveterconfigured from double gate MOSFET and method of fabricating same.”

The multiple-gate transistor structures described, i.e., the double-gatetransistor 100, the triple-gate transistor 102, and the omega-FET 104,have a common feature: the fin-like semiconductor active region 112. Indoping the source and drain regions 126 and 128 of the semiconductorfin, prior art uses a conventional source and drain ion implantationprocess where ions are implanted at a small angle with respect to thenormal of the wafer or the substrate. In conventional source and drainimplantation, a small angle of 7 degrees or less is frequently used, asshown in FIG. 4 a, which is taken along the B-B′ line of FIG. 2.

Using such an ion implantation condition results in most of theimplanted dopants reaching the top surface 124 of the fin 112, giving ahigh-doped top surface region 132. Few dopants are effectively implantedinto the sidewall surface 114, resulting in a lightly doped sidewallsurface region 134. A long implantation time may be needed to introducea significant amount of doping in the sidewall source/drain regions126/128. In addition, since less dopants reach the bottom portion of thefin, the source and drain doping at the bottom portion of the fin willbe lower, and may result in a larger channel length at the bottomportion of the fin than at the top portion of the fin.

FIG. 4 b shows a cross-section of FIG. 2 taken along the C-C′ line. Avariation of the channel length of channel 136 within the fin 112, i.e.,a larger channel length at the bottom portion of the fin, results in anon-uniform source-to-drain current distribution with reduced current atthe bottom portion of the fin. Therefore, the performance of thesemiconductor fin device is not optimized when the conventional sourceand drain doping process is used.

Embodiments of the present invention can be implemented using any of anumber of multiple gate transistors. Three examples of these transistorsare described with respect to FIGS. 1-4. Any of these structures, aswell as other structures, can utilize concepts of the present invention.As illustrated in a cross-sectional view in FIG. 1 a, a double-gatetransistor 100 has a gate electrode 110 that straddles across thechannel or the fin-like silicon body 112, thus forming a double-gatestructure. There are two gates, one on each sidewall 114 of the siliconfin 112, as shown in FIG. 1 a. The plan view of the double-gatestructure is shown in FIG. 2, where FIG. 1 a (and 1 b and 1 c) are takenalong the A-A′ line of FIG. 2. The silicon fin 112 is formed on aninsulator 116 which is formed on silicon substrate 118. The silicon body112 is separated from the gate electrode 110 by a gate dielectric 120along the sidewalls 114 and by a mask 122 along a top surface of the fin114.

Examples of double-gate transistors are provided in the followingreferences, each of which is incorporated herein by reference. As willbecome clear from the teachings below, the structures disclosed in thesereferences can be modified and/or utilize the methods of the presentinvention.

X. Huang, et al., “Sub-50 nm p-channel finFET,” IEEE Trans. ElectronDevices, vol. 48, no. 5, pp. 880-886, May 2001.

C. Hu et al., U.S. Pat. No. 6,413,802, “FinFET transistor structureshaving a double gate channel extending vertically from a substrate andmethods of manufacture,” Jul. 2, 2002.

F. L. Yang, et al., “35 nm CMOS FinFETs,” Symposium on VLSI Technology,Digest of Technical Papers, pp. 109-110, June 2002.

H. S. P. Wong, “Beyond the conventional transistor,” IBM J. Research andDevelopment, vol. 46, no. 2/3, pp. 133-168, March/May 2002.

F. L. Yang et al., U.S. Pat. No. 6,252,284, “Planarized Si fin device,”Jun. 26, 2001.

B. Yu, U.S. Pat. No. 6,391,695, “Double-gate transistor formed in athermal process,” May 21, 2002.

Multiple-gate transistors such as the double-gate transistor, thetriple-gate transistor, the omega-FET, have a common feature: thesemiconductor fin-like active region. Therefore, such devices are alsoknown as semiconductor fin devices. The semiconductor fin has apredetermined fin height h and a predetermined fin width w. Anothercommon feature of multiple-gate transistors is that the sidewallsurfaces of the semiconductor fins are used for current conduction,i.e., a significant amount of source-to-drain current in themultiple-gate transistor is carried along the sidewall surfaces.

Essentially, the effective device width of the multiple-gate transistoris a function of the fin height h (see FIG. 1 a). In the double-gatetransistor, the device width is twice the fin height, i.e., 2h. In thetriple-gate transistor, the device width is given by (2h+w) (see FIGS. 1a and 1 b). As shown in FIGS. 3 a and 3 b, portions of the sidewalls ofthe semiconductor fin 112 are doped for forming the source region 126and the drain region 128. Therefore, in the fabrication of semiconductorfin devices or multiple-gate transistors, processes involving the dopingof the semiconductor fin, and particularly the sidewall surfaces of thesemiconductor fin, are used. Effective and efficient doping of thesidewall surfaces of the semiconductor fin can be used to optimize thetransistor characteristics. In one aspect, this invention providesimproved methods of doping the semiconductor fin sidewalls in amanufacturing process.

According to this aspect of the invention, the implant angle α for thesource and drain doping of the semiconductor fin device is large tooptimize the device performance. Referring now to FIGS. 5 a-5 c, an ionimplantation scheme is shown where the ion implantation is performed inat least two implant steps. As shown in FIG. 5 a, a semiconductor fin112 is provided on an insulator layer 116. The insulator layer 116 mayhave a recess as in the case of an omega-FET as shown in FIG. 1 c. Thethickness of the insulator layer may range from about 100 angstroms toabout 2000 angstroms. For illustration purposes, we consider thesemiconductor fin 112 for a triple-gate transistor 102.

In the preferred embodiment, the semiconductor fin 112 is formed fromsilicon, and the implanted ions are p-type dopant ions such as boronand/or indium or n-type dopant ions such as phosphorus, arsenic, and/orantimony. In the first implant step, as shown in FIG. 5 b, an angledimplant (denoted by arrows 140) with an angle α with respect to thenormal of the wafer top surface 124 in the z-x plane is performed. Theimplanted dose is about half the total dose I (in units of dopants perunit wafer surface area) of dopants for the source 126 and drain 128 ionimplantation. The total dose I for the source and drain ion implant istypically in the range of about 1×10¹³ to about 1×10¹⁶ dopants persquare centimeter. The first implantation step 140 creates doped topregion 144 and doped sidewall region 146.

At the point of incidence at the top fin surface 124, the ions approachthe top surface 124 at an angle α with respect to the normal of the topfin surface. The normal of the top fin surface is typically parallel tothe normal of the wafer. At the point of incidence to the fin sidewallsurface 114, the ions approach the sidewall surface 114 at an angle of(90−α) degrees with respect to the normal of the fin sidewall surface.The angles of a and (90−α) are measured in the plane perpendicular tothe plane of the sidewall surface 114.

In the first implant step, the fin's first sidewall surface 114 receiveda dose of about (I/2).sin(α), the fin's top surface 124 received a doseof about (I/2).cos(α), and the fin's second sidewall surface 115received essentially no dopants.

In the second implant step, the device 102 is rotated 180 degrees aboutits normal and the second half dose is implanted at an angle α withrespect to the normal of the wafer in the z-x plane, as shown in FIG. 5c. In the second implant step (denoted by arrows 142), the fin's secondsidewall surface 115 received a dose of about (I/2).sin(α), the fin'stop surface 124 received another dose of about (I/2).cos(α), and fin'sfirst sidewall surface 114 received essentially no dopants. As a result,after the first and the second implant steps, the total dose received bythe top surface of the fin is I.cos(α), and the total dose received byeach sidewall surface is (I/2).sin(α). FIG. 5 c illustrates theformation of doped sidewall region 148.

In the embodiment illustrated in FIGS. 5 b and 5 c, both ionimplantation steps 140 and 142 were performed at an angle α. It is notedthat the angle of the first implantation step need not be equal to theangle of the second implantation step. It is desirable, (but notnecessary) that these angles be close in value so as to maintainconsistent operating characteristics in sidewall doped regions 146 and148.

In FIG. 6 a, the dose of dopants per unit surface area received in a topsurface region 144 and in a sidewall surface region 146 (or 148) of thefin 112 are plotted as a function of the implant angle α. If the angle αis zero, (I/2).sin(α) becomes zero, i.e., no dopants will reach the finsidewall surface, while I.cos(α) becomes I, i.e., the top surface of thefin will be doped with a dose I. Consequently, if a zero implant angleis used, the source and drain regions 126 and 128 on the fin sidewalls114 and 115 of the multiple-gate transistor cannot be effectivelyformed. It is seen that in conventional source and drain ion implantconditions using small angles such as 7 degrees, the dose per unitsurface area received by each sidewall surface is less than a tenth ofthe dose per unit surface area received by the top surface. In thiscase, a very high implant dose and long implantation time are requiredfor the fin sidewall surface to receive a substantial amount of doping.

The preferred embodiment of this invention teaches that the implantangle should be large for the sidewall surfaces 114 and 115 to receive asubstantial amount of doping. In fact, the implant angle is as large as60 degrees for the top and sidewall surfaces of the fin 112 to havecomparable doses, resulting in comparable doping concentrations.

Referring now to FIG. 6 b, the ratio of the dose received on the top finsurface 124 to the dose received on a fin sidewall surface 114 (or 115)is plotted as a function of the implant angle α. This ratio can also beinterpreted as the ratio of the doping concentration in the top finsurface region 144 to the doping concentration in a fin sidewall surfaceregion 146 (or 148). In one embodiment of the present invention, theratio is preferably less than about 8, and the corresponding implantangle therefore is more than about 15 degrees. In the preferredembodiment, the ratio is in the range of about 1 to about 4, and thecorresponding implant angle has to be in the range of about 26 degreesto about 63 degrees. The doping concentrations in the top surface region144 and the sidewall surface regions 146 and 148 are preferably morethan about 1×10²⁰ dopants per cubic centimeter.

According to the teaching of this invention, a large implant angle α hasa number of advantages. First, a large angle implant can introduce moredopants more efficiently in the source and drain regions 126 and 128 onthe sidewall surfaces of the transistor. As a result, the use of a smallangle implant with long implantation time can be avoided. Second, alarge angle implant will dope the top surfaces 124 and the sidewallsurfaces 114 and 115 of the source and drain regions in the fin moreequally. As a result, the channel length of the multiple-gate transistoris maintained the same whether at the top portion of the fin or at thebottom portion of the fin.

However, potential problems arise in the use of large angle implants,and solutions to these problems will be provided according to aspects ofthe present invention. Referring now to FIG. 7 a, a plan view of twomultiple-gate transistors 102 a and 102 b are shown, where the twotransistors 102 a and 102 b have source-to-drain directions that areperpendicular to each other, i.e., the semiconductor fins 112 areoriented in directions that are perpendicular to each other. In otherwords, the orientations of the semiconductor fins are orthogonal to eachother. In FIG. 7 a, the source-to-drain direction of the firsttransistor 102 a is in the y-direction, and the source-to-draindirection of the second transistor 102 b is in the x-direction. The x-yaxes are shown in the bottom right corner of FIG. 7 a.

The orientations of the gate electrodes 110 a and 110 b of the twotransistors are also perpendicular to each other. It is understood thatthe two transistors may be in close proximity to each other, asillustrated in FIG. 7 a, or may be far apart from each other, such asbeing located at extreme ends of an integrated circuit die. Themultiple-gate transistors shown in FIG. 7 a may be double-gatetransistors, triple-gate transistors, or omega-FETs. For illustrationpurposes, the multiple-gate transistors are triple-gate transistors.

FIG. 7 b shows the cross-sectional view of the triple-gate transistors102 a and 102 b along line A-A′ of FIG. 7 a. Line A-A′ of FIG. 7 a cutsthrough the source region 126 a of the first transistor 102 a and thesource region 126 b, channel region 136 b, and drain region 128 b of thesecond transistor 102 b. In general, in the fabrication process ofmultiple-gate transistors, semiconductor fins 112 are provided on aninsulator layer 116, a gate dielectric layer 120 is deposited, followedby the formation of gate electrodes 110.

The gate dielectric layer may be comprised of silicon oxide or siliconoxynitride. The gate dielectric layer may also comprise highpermittivity dielectrics such as lanthalum oxide (La₂O₃), aluminum oxide(Al₂O₃), hafnium oxide (HfO₂), hafnium oxynitride (HfON), zirconiumoxide (ZrO₂), or combinations thereof. High permittivity dielectricstypically have a relative permittivity of greater than 5. The gateelectrode may be comprised of a conductive material. Examples ofconductive materials are doped poly-crystalline silicon, dopedpoly-crystalline silicon-germanium, a metal, or a metal silicide. Atthis point, before the formation of the source and drain regions 126 and128, the channel region 136 in the semiconductor fin 112 may be undopedor doped.

The subsequent fabrication process step may involve the formation of thesource region 126 and drain region 128 by doping appropriate portions ofthe semiconductor fin 112. If a large angle implant with an angle α inthe z-x plane is used to dope the first sidewall 114 a of thesemiconductor fin of the first transistor 102 a, the channel region 136b of the second transistor 102 b will also receive the source and draindopants, causing degradation to the performance of the second transistor102 b. The same problem occurs when using a large angle implant to dopethe second sidewall 115 a of the first transistor 112 a.

In general, the large angle implant steps, while providing efficientdoping of the source and drain regions of transistors with a firstsource-to-drain orientation, e.g., source-to-drain direction iny-direction, will also dope the channel region of the transistors with asecond perpendicular source-to-drain orientation, e.g., source-to-draindirection in x-direction, with the same source/drain dopants. This isbecause a source and drain implant 140 with a large implant angle α suchas 30 degrees has an implant angle similar to that of a halo implant andcan therefore dope the channel region 136 b of the second transistor.Standard conventional halo implants, however, dope the channel region136 with dopants of the opposite type to the source 126 and drain 128dopants in order to control short channel effects. When the channelregion 136 b of the second transistor 102 b is doped with the dopantsintended for the source and drain regions 126 a and 128 a, the secondtransistor 102 b will have degraded short-channel behavior and may evenfail due to an electrical short between the source and drain 126 b and128 b.

According to aspects of this invention, a method is provided in which alarge angle implant may be used to dope the source and drain regions 126a and 128 a of the first transistor 102 a with a first source-to-drainorientation without doping the channel region 136 b of the secondtransistor 102 b with a second perpendicular source-to-drainorientation. Prior to the source and drain implant, a mask material 150is deposited on the wafer to cover both the first and second transistors102 a and 102 b. The mask material is patterned, e.g., by opticallithography, and the portion of the mask material 150 that covers thefirst transistor 102 a is removed to form a first mask 150, as shown inFIG. 8 a. The first mask 150 covers the second transistor 102 b,shielding the channel region 136 b, during the doping of the source anddrain regions 126 a and 128 a of the first transistor 102 a.

The doping of the source and drain regions 126 a and 128 a may beperformed in two implant steps, as described above, in which a half-doseangled implant is performed to dope the first sidewall 114 a and topsurface 124 a of the fin 112 a of the first transistor 102 a followed byanother half-dose angled implant to dope the second sidewall 115 a andtop surfaces 124 a. Following the doping of the source and drain regions126 a and 128 a, the first mask 150 may be removed.

Next, a second mask 152 is formed to expose the second transistor 102 bwhile covering the first transistor 102 a. An ion implantation includingat least two steps is performed to dope the source and drain regions 126a and 128 a of the second transistor 102 b. The channel region 136 a ofthe first transistor 102 a is shielded from this ion implantationprocess by the second mask 152. The implant angle of the ionimplantation process to dope the source and drain regions 126 b and 128b of the second transistor 102 b is oriented at an angle β in the z-yplane. The angle β is in the z-y plane which is perpendicular to theplane of the sidewall 114 b of the semiconductor fin 112 b of the secondtransistor. In FIG. 8 b, the positive y-axis is into the plane of thepaper while the negative y-axis is out of the plane of the paper (and,as a result, the angle β cannot be explicitly drawn).

The second mask 152 may then be removed. A high temperature annealingstep may be performed to activate the implanted dopants in thesemiconductor fins 112 a and 112 b. The annealing step may be a spikeanneal process in which the wafer temperature is rapidly increased to apeak temperature of 1050 degrees Celsius followed by a rapid cooling ofthe wafer temperature, or any other annealing techniques such as a rapidthermal anneal (RTA) commonly known and used in the art.

In the above-mentioned method embodiment, it is seen that the doping ofsource and drain regions 126 and 128 in transistors of the same type,e.g., n-type or n-channel, involves an additional mask patterning step.The introduction of an additional mask is sometimes costly and could becommercially prohibitive. Therefore, a further improvement over theabove-mentioned embodiment is to align all multiple-gate transistors ofthe same type with a predetermined range of gate lengths in the samedirection to permit the use of a large angle implant without theintroduction of an additional mask. This is described in anotherembodiment of the present invention.

According to this embodiment, all multiple-gate transistors of the sameconductivity type and with gate lengths less than or equal to apredetermined gate length are oriented in the same direction. In thiscontext, all of transistors refers to all of the functional oroperational transistors that are designed to operate with optimumcharacteristics. For example, this does not include dummy transistors orother transistors that do not operate in the circuits of the chip. Thepredetermined gate length is determined based on susceptibility toshort-channel effects. Transistors with shorter gate lengths are moresusceptible to short-channel effects. The predetermined gate length maybe 30 nm, for example. In another example, the predetermined gate lengthmay be the minimum gate length.

Since the large angle implant for doping the source and drain regions126 and 128 of transistors of a first source-to-drain orientationdegrade short-channel effects of transistors with other source-to-drainorientations, transistors susceptible to degradation of short-channeleffects should all have the same source-to-drain direction. This means,for example, that all n-type multiple-gate transistors with gate lengthsless than 30 nm have the source-to-drain direction oriented in thex-direction. This is illustrated in FIG. 9 where gate lengths L_(g,a)and L_(g,b) are less than the predetermined gate length, e.g., 30 nm,while L_(g,c) and L_(g,d) are larger than the predetermined gate length.Transistor 102 a and transistor 102 b of FIG. 9, having gate lengths ofL_(g,a) and L_(g,b) respectively, therefore have substantially the samesource-to-drain orientations (i.e., the source-to-drain current pathsare substantially parallel). Transistor 102 c and transistor 102 d havegate lengths of L_(g,c) and L_(g,d), respectively, and they may have anysource-to-drain orientations.

It is understood, however, that transistor 102 c and transistor 102 dmay have different electrical characteristics or vulnerability toshort-channel effects depending on the ion implantation conditions usedto dope their source and drain regions 126 and 128. The x-direction canbe, for example, a crystallographic direction such as the [100]direction. In this case, the sidewall surfaces of the n-channelmultiple-gate transistor are (100) surfaces. N-channel multiple-gatetransistors with (100) sidewall surfaces are expected to have the bestelectron mobility. In another example, all p-type multiple-gatetransistors with gate lengths less than 30 nm may have thesource-to-drain direction oriented in the [110] crystallographicdirection. In this case, the sidewall surfaces of the p-channelmultiple-gate transistor are (110) surfaces. P-channel multiple-gatetransistors with (110) sidewall surfaces is expected to have the besthole mobility.

In another method embodiment, the semiconductor fins 112 may be doped bysolid-source diffusion instead of ion implantation as described in theother method embodiments. In the solid-source diffusion technique, adopant-containing material (the solid source) is deposited on thesemiconductor fin to be doped. An elevated temperature treatment is thenperformed to allow the dopants in the dopant-containing material orsolid-source to diffuse into the semiconductor fins. Examples ofdopant-containing materials include boro-silicate glass (BSG),phospho-silicate glass (PSG), doped germanium, etc. The discussionrelated to FIG. 9 also applies to this embodiment.

While several embodiments of the invention, together with modificationsthereof, have been described in detail herein and illustrated in theaccompanying drawings, it will be evident that various modifications arepossible without departing from the scope of the present invention. Theexamples given are intended to be illustrative rather than exclusive.The drawings may not necessarily be to scale and features may be shownin a schematic form.

1. A method of doping semiconductor fins of multiple-gate transistors,the method comprising: providing a semiconductor structure comprising ofa plurality of semiconductor fins overlying an insulator layer, each finhaving a gate dielectric overlying a portion of that semiconductor fin,and a gate electrode overlying said gate dielectric, each of thesemiconductor fins having a channel region disposed beneath the gateelectrode, the channel region located in a top surface, a first sidewallsurface, and a second sidewall surface of the semiconductor fin;implanting dopant ions at an angle α with respect to the normal of thetop surface of each of the semiconductor fins to dope along the firstsidewall surface and along the top surface, the angle α being between 26degrees and about 63 degrees; and implanting dopant ions at an angle βwith respect to the normal of the top surface of each of thesemiconductor fins to dope along the second sidewall surface and alongthe top surface, the angle β being between 26 degrees and about 63degrees.
 2. The method of claim 1 wherein the angle α has a magnitudethat is about equal to the magnitude of angle β.
 3. The method of claim1 further comprising an annealing process to activate the implanteddopant ions.
 4. The method of claim 1 wherein the semiconductor fins areoriented in the same direction.
 5. The method of claim 1 wherein eachgate electrode has a gate length smaller than 30 nm and eachsemiconductor fin is oriented in the same direction.
 6. The method ofclaim 1 wherein the semiconductor fins comprise silicon fins.
 7. Themethod of claim 6 wherein the dopant ions are p-type dopant ionsselected from the group consisting of boron and indium and combinationsthereof.
 8. The method of claim 7 wherein the first and second sidewallsurfaces are (110) crystallographic surfaces.
 9. The method of claim 6wherein the dopant ions are n-type dopant ions selected from the groupconsisting of phosphorus, arsenic, and antimony and combinationsthereof.
 10. The method of claim 9 wherein the first and second sidewallsurfaces are (100) crystallographic surfaces.
 11. The method of claim 1wherein each semiconductor fin comprises an etchant mask overlying thesemiconductor fin.
 12. The method of claim 1 wherein the insulator layeris recessed, resulting in a notch at the base of each of thesemiconductor fins.
 13. The method of claim 1 wherein the ratio of thedoping concentration in a top surface region of the semiconductor fin tothe doping concentration in the first sidewall region of thesemiconductor fin is between about 1 and about
 4. 14. The method ofclaim 1 wherein the first and second sidewall regions are doped to adoping concentration of more than 1×10²⁰ cm⁻³.
 15. The method of claim 1wherein the insulator layer has a thickness of between about 100angstroms and about 2000 angstroms.
 16. The method of claim 1 whereinthe gate dielectric layer comprises high permittivity material.
 17. Themethod of claim 16 wherein the high permittivity material is selectedfrom the group consisting of lanthanum oxide, aluminum oxide, hafniumoxide, hafnium oxynitride, zirconium oxide, and combinations thereof.18. The method of claim 16 wherein the gate dielectric layer comprisesof high permittivity materials with relative permittivity greater than5.
 19. The method of claim 1 wherein the gate electrode comprisespoly-crystalline silicon.
 20. The method of claim 1 wherein the gateelectrode comprises a metal.
 21. A method of forming asemiconductor-on-insulator chip, the method comprising: providing asubstrate with an insulator layer formed thereon; forming a plurality ofmultiple-gate transistors on the insulator layer, wherein at least someof the multiple-gate transistors are formed by: forming a semiconductorfin having an orientation in a first direction; forming a gate electrodeformed adjacent a channel region portion of the semiconductor fin, thegate electrode having a gate length of less than about 30 nm; forming asource region and a drain region within the semiconductor fin such thatthe channel region is disposed between the source region and the drainregion, the channel extending along a first sidewall surface of the fin,across a top surface of the fin and along a second sidewall surface ofthe fin, wherein the channel region is doped to a first conductivitytype and the source and drain regions are doped to a second conductivitytype that is different than the first conductivity type, wherein thesource and drain regions are doped by implanting ions at an anglebetween 26 degrees and about 63 degrees with respect to the normal of atop surface of the insulator layer.
 22. The method of claim 21 whereinthe semiconductor fin is comprised of silicon, and wherein theorientation of the semiconductor fin is in the [110] direction.
 23. Themethod of claim 21 wherein the semiconductor fin is comprised ofsilicon, and wherein the orientation of the semiconductor fin is in the[100].
 24. A method of doping semiconductor fins of multiple-gatetransistors, the method comprising: providing a semiconductor structurecomprising of a plurality of semiconductor fins overlying an insulatorlayer, each fin having a gate dielectric overlying a portion of thatsemiconductor fin, and a gate electrode overlying said gate dielectric,each of the semiconductor fins having a top surface, a first sidewallsurface, and a second sidewall surface; implanting dopant ions at anangle α with respect to the normal of the top surface of each of thesemiconductor fins to dope a portion of the semiconductor fins, theangle α having a magnitude between 26 degrees and 63 degrees; andimplanting dopant ions at an angle β with respect to the normal of thetop surface of each of the semiconductor fins to dope a portion of thesemiconductor fins, the angle β having a magnitude between 26 degreesand 63 degrees.
 25. The method of claim 24 wherein implanting dopantions at an angle α comprises doping the first sidewall surface and thetop surface of the semiconductor fins and wherein implanting dopant ionsat an angle β comprises doping the second sidewall surface and the topsurface of the semiconductor fins.